 When the flow is in the upper part of the atmospheric boundary layer, which is a kilometer or two above the surface, the turbulence in the boundary layer acts to impede the horizontal wind. This resistance to flow is not really friction, but it does act to slow the wind down, no matter the wind's direction. As a result, we can assume that this turbulent drag is a force that opposes the wind velocity. Look at what adding a turbulent resistant term does to the force balance for straight line flow in the upper boundary layer. The PGF force is perpendicular to the pressure gradient, as usual. However, the friction opposes the velocity and slows it down. At the same time, the Coriolis force is always perpendicular to the velocity and to the right. And because the velocity is slowed down, the Coriolis force is less. The velocity vector gets turned toward the PGF vector and thus toward the low pressure. Note that the x-direction of the frictional force must balance in the opposite to the x-direction of the Coriolis force. And the y-directions of the friction in Coriolis forces must be opposite to and balance the PGF in the y-direction in the diagram. Boundary layer turbulent drag turns the velocity across isobars toward low pressure, which causes convergence, while boundary layer turbulent drag turns the velocity across isobars away from high pressure, which causes divergence. These friction effects tend to amplify the convergence into surface low pressure and divergence from surface high pressure.